[PATCH] drm: bridge/dw_hdmi: Filter modes > 165MHz for DVI

[Doug Anderson]

Russell,

On Wed, Jun 17, 2015 at 4:30 PM, Russell King - ARM Linux
<linux at arm.linux.org.uk> wrote:
> On Wed, Jun 17, 2015 at 04:14:07PM -0700, Doug Anderson wrote:
>> If you plug in a DVI monitor to your HDMI port, you need to filter out
>> clocks > 165MHz.  That's because 165MHz is the maximum clock rate that
>> we can run single-link DVI at.
>>
>> If you want to run high resolutions to DVI, you'd need some type of an
>> active adapter that pretended that it was HDMI, interpreted the
>> signal, and produced a new dual link DVI signal at a lower clock rate.
>>
>> Signed-off-by: Doug Anderson <dianders at chromium.org>
>> ---
>> Note: this patch was tested against a 3.14 kernel with backports.  It
>> was only compile tested against linuxnext, but the code is
>> sufficiently similar that I'm convinced it will work there.
>
> Really?  I have to wonder what your testing was...
>
>         hdmi->vic = drm_match_cea_mode(mode);
>
>         if (!hdmi->vic) {
>                 dev_dbg(hdmi->dev, "Non-CEA mode used in HDMI\n");
>                 hdmi->hdmi_data.video_mode.mdvi = true;
>         } else {
>                 dev_dbg(hdmi->dev, "CEA mode used vic=%d\n", hdmi->vic);
>                 hdmi->hdmi_data.video_mode.mdvi = false;
>         }
>
> mdvi indicates whether the _currently set mode_ is a CEA mode or not (imho,
> it's mis-named).  It doesn't indicate whether we have a HDMI display device
> or a DVI display device connected, which seems to be what you want to use
> it for below.
>
> To sort that, what you need to do is detect a HDMI display device using
> drm_detect_hdmi_monitor() on the EDID received from the device before
> parsing the modes, and save that value in a dw_hdmi struct member, and
> I'd suggest that it's a top-level struct member, not buried in 'hdmi_data'
> or 'video_mode'.

OK, so clearly my patch won't work against mainline.  I guess it's a
good thing that I pointed out that it was only tested locally (would
have been better to test against mainline, but I don't think that's so
easy since there are several unlanded patches in mainline for
Rockchip).

As pointed out by others at <http://crosreview.com/278255>, locally
our kernel has a slightly older version of
<https://lkml.org/lkml/2015/2/28/291>, which would change mdvi to be
as needed.

...so I guess my change is blocked on someone reviewing/landing that
series.  If that series is rejected (or is changed sufficiently so
that mdvi no longer is set via drm_detect_hdmi_monitor() then my patch
will need to be re-spun.

-Doug